
where we used the fact that \(c(-k)=-c(k)\) . We now use the following result [1]}:
Let \(g(k)\)  be complex valued, \(I(k)\)  be real valued functions
of the real variable \(k\) . Let \(t>0\)  and define the integral 
\(F(t)=\int _{-\infty }^{\infty }dk\,g(k)\,e^{\imath t\,I(k)}.\) 
